Orchidaceae is the largest family of plants, reaching its maximum diversity in Colombia where 4000 species have been registered. One particular ecosystem with high diversity of orchids is the tropical montane cloud forest characterized by high humidity and low air temperatures. However, due to anthropic pressure such as land use change its area has been reduced. This is not the only anthropic disturbance that can affect the continuity of this forest and orchids, climate change and nitrogen deposition also become threats. By means of elemental composition and isotopic measurements of carbon and nitrogen, we determined the photosynthetic pathway for orchids from a tropical cloud forest in Colombia, for which, we also evaluated the nutritional status of nitrogen and its relationship with atmospheric deposition. We found 46 species from 18 genera; of these, 19 species were epiphytic, 9 lithophytic and 18 terrestrials. The carbon content was not significantly different among the species evaluated, averaging 46.4 ± 0.41% (dry weight). In turn, only one terrestrial orchid had a nitrogen content above 2.2%, averaged 1.4 ± 0.07% for the remaining 45 species. The δ¹³C that averaged − 28.1 ± 0.4‰ for 45 species was typical of C₃ plants, while only one species, whose δ¹³C of 18.8‰ can be considered CAM. The δ¹⁵N values for 44 orchids were negative, in total they ranged from − 8.1 to 4.5‰. It can be concluded that the temperatures in this area are not yet high enough to produce a noticeable change in photosynthesis as evidenced by the carbon content of orchids, however, the expected rise would bring harmful consequences for these populations of orchids, especially when considering that these orchids were collected at the highest altitudinal range of the region. Moreover, the rates of nitrogen deposition are not high enough yet to leave a significant signal in the vegetation.
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